THE BP oil spill left an oily “bathtub ring” on the sea floor that’s about the size of Rhode Island, new research shows.

The study by David Valentine, the chief scientist on the federal damage assessment research ships, estimates that about 10 million gallons of oil coagulated on the floor of the Gulf of Mexico around the damaged Deepwater Horizon’s oil rig.

Spread ... Thick oil from the BP Deepwater Horizon oil spill floats on the surface of the water and coats the marsh wetlands in Bay Jimmy near Port Sulphur, Louisiana. Picture: AFP PHOTO/Saul LOEBSource:AFP

Prof Valentine, a geochemistry professor at the University of California Santa Barbara, said the spill from the Macondo well left other splotches containing even more oil. He said it is obvious where the oil is from, even though there were no chemical signature tests because over time the oil has degraded.

“There’s this sort of ring where you see around the Macondo well where the concentrations are elevated,” Prof. Valentine said. The study, published in Monday’s Proceedings of the National Academy of Sciences, calls it a “bathtub ring.”

Oil levels inside the ring were as much as 10,000 times higher than outside the 3100-square-kilometres ring, Prof. Valentine said. A chemical component of the oil was found on the sea floor, anywhere from one kilometre to 1.6 kilometres below the surface.

The rig blew on April 20, 2010, and spewed 172 million gallons of oil into the Gulf through the summer. Scientists are still trying to figure where all the oil went and what effects it had.

BP questions the conclusions of the study.

How it happened ... Video provided by BP PLC, shows oil leaks from the fractured wellhead at the site of the Deepwater Horizon oil spill in the Gulf of Mexico. Picture: BPSource:AP

In an email, spokesman Jason Ryan said, “the authors failed to identify the source of the oil, leading them to grossly overstate the amount of residual Macondo oil on the sea floor and the geographic area in which it is found.”

It’s impossible at this point to do such chemical analysis, said Prof. Valentine and study co-author Christopher Reddy, a marine chemist at Woods Hole Oceanographic Institute, but all other evidence, including the depth of the oil, the way it laid out, the distance from the well, directly point to the BP rig.